Phenylpropanoids are plant-derived organic compounds that are biosynthesized from the amino acid phenylalanine. Intermediates and end products of this pathway include compounds having important roles in plants, such as phytoalexins, antiherbivory compounds, antioxidants, ultra-violet protectants, pigments, and aroma compounds. The majority of the carbon in the pathway, however, is channeled toward the synthesis of lignin. As the second most abundant polymer on earth, exceeded only by cellulose, lignin is a major carbon sink in the biosphere, accounting for about 30% of the carbon sequestered into terrestrial plant material each year (Battle et al., Science, 287:2467 (2000)).
Lignin is a major structural component of secondarily thickened cell walls of tissues with conducting and/or mechanical functions. Angiosperm lignin is composed of three main units named p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units. These components originate from the polymerization of three monolignols, p-coumaryl, coniferyl, and sinapyl alcohols, respectively. The monolignols are synthesized from phenylalanine through successive deamination, reduction, hydroxylation, and methylation steps. The proportions of H, G, and S units in the cell wall vary according to plant species and tissue type.
As a major polymer of cell walls, lignin has a direct impact on the characteristics of plants and plant products, such as wood. Highly lignified wood is durable and therefore a good raw material for many applications. Since lignin yields more energy when burned than cellulose, lignified wood is also an excellent fuel. The mechanical support provided by lignin prevents lodging, a problem in many agronomically important plants. On the other hand, lignin is detrimental to paper manufacture and must be removed from pulp before paper can be manufactured. This is costly both in terms of energy and the environment.
Lignin also makes it difficult to break down biomass for conversion into cellulosic ethanol biofuel. Cellulosic ethanol, which exhibits a net energy content three times higher than corn ethanol, can be produced from a wide variety of cellulosic biomass feedstocks including agricultural plant wastes, plant wastes from industrial processes and energy crops grown specifically for fuel production. Cellulosic biomass is composed largely of cellulose, hemicellulose and lignin, with smaller amounts of proteins, lipids and ash. Processing cellulosic biomass aims to extract fermentable sugars from the feedstock, which requires disruption of the hemicellulose/lignin sheath that surrounds the cellulose in plant material. Technological developments that increase the yield and drive down the production cost of cellulosic ethanol can help to reduce our oil dependency in a sustainable way. Given the role of lignin in the recalcitrance of biomass for conversion to biobased fuels, in addition to the many other roles of lignin, it is desirable to have the ability to produce plants with modulated levels of lignin.